1. Field of the Invention
Implementations consistent with the principles of the invention generally relate to the field of battery technology, more specifically to electrodes, such as anodes or negative electrodes, for three-dimensional lithium batteries and their methods of manufacture.
2. Background
Existing energy storage devices, such as batteries, fuel cells, and electrochemical capacitors, typically have planar architectures with an actual surface area of each component being roughly equivalent to a geometrical area, with a porosity being responsible for any area increase over the geometrical area. Energy storage devices such as lithium batteries are the state of the art power sources for many electronic devices due to their high energy density, high power, and long shelf life.
FIG. 1 shows a cross sectional view of an existing energy storage device, such as a lithium-ion battery. The battery 15 includes a cathode current collector 10, on top of which a cathode 11 is assembled. This layer is covered by a separator 12, over which an assembly of an anode current collector 13 and an anode 14 are placed. This stack is then sometimes covered with another separator layer (not shown) above the anode current collector 13, and is then rolled and stuffed into a can to assemble the battery 15. During a charging process, lithium leaves the cathode 11 and travels through the separator 12 as a lithium ion into the anode 14. Depending on the anode 14 used, the lithium ion either intercalates (e.g., sits in a matrix of an anode material without forming an alloy) or forms an alloy. During a discharge process, the lithium leaves the anode 14, travels through the separator 12 and passes through to the cathode 11.
Anodes for lithium ion batteries generally fall into two categories: 1) anodes that hold lithium within a material matrix, which are referred to as intercalation anodes; and 2) anodes that form an alloy in the presence of lithium, which are referred to as alloy anodes. Carbon is an example of a material for forming intercalation anodes, while aluminum, silicon, and tin are examples of materials for forming alloy anodes.
In the process of formation of a Li—X alloy (where X is a material that can form an alloy with lithium), there can be a significant volume difference between an alloyed and an un-alloyed state. In particular, the alloyed state can occupy a significantly greater volume than the un-alloyed state. In other words, alloy anodes can change volume by a significant fraction during every charge-discharge cycle. This can pose a significant problem for the stability and cycle life of the anodes when incorporated into batteries. In particular, alloy anodes can have capacity loss by way of cracks that are formed during volume change. During repeated cycling, these cracks can propagate and cause parts of an anode material to separate from a matrix. This can cause a decrease in the amount of the anode material that is electrically connected to a current collector, thereby causing capacity loss. In some instances, the volume change in alloy anodes can be as high as 300%. Certain methods have been proposed to overcome the problems of capacity loss due to expansion and contraction of alloy anodes. Unfortunately, these methods suffer from a number of deficiencies, and often involve a traditional planar architecture for a battery.
Three-dimensional batteries have been proposed in the literature as ways to improve battery capacity and active material utilization. It has been proposed that a three-dimensional architecture can be used to provide higher surface area and higher energy as compared to a two-dimensional, flat battery architecture.
FIG. 2A illustrates one possible design for a structured silicon anode that is assembled into a lithium-ion battery with a planar cathode in a discharged state, as has been proposed in the literature. For example, reference to Green et al., “Structured Silicon Anodes for Lithium Battery Applications,” Electrochemical and Solid State Letters, 6, 2003 A75-A79, may help to illustrate the state of the art in structured silicon anodes, and is therefore incorporated by reference as non-essential subject matter herein. Referring to FIG. 2A, a cathode sheet including a cathode current collector 20 along with a cathode active porous material 21 is assembled on top of a separator material 22. This dual-layered material is then attached to a structured silicon anode material 23, which is in the form of pillars that are connected to an anode current collector 24. During charging, lithium ion transport occurs from the cathode active material 21 through the separator material 22 into the anode material 23. Since the anode material 23 in this case is made out of silicon, the charging process expands it. As can be seen in FIG. 2B, top portions of the anode material 23, which are geometrically closer to the cathode active material 21 than bottom portions of the anode material 23, experience larger amounts of expansion. This non-uniform expansion can cause a non-uniform current density and, thereby, a non-uniform capacity utilization. This is pictorially shown in FIG. 2B, where the top portions of the anode material 23 are in an expanded state due to preferential alloying. In certain cases, the top portions can close off before the bottom portions can be lithiated.
The following references may also help to illustrate the state of the art, and are therefore incorporated by reference as non-essential subject matter herein: Shin et al., “Porous Silicon Negative Electrodes For Rechargeable Lithium Batteries,” Journal of Power Sources, 139 (2005) 314-320; Long et. al., “Three-Dimensional Battery Architectures,” Chemical Reviews, (2004), 104, 4463-4492; Broussely and Archdale, “Li-ion batteries and portable power source prospects for the next 5-10 years,” Journal of Power Sources, 136, (2004), 386-394; Canadian Patent CA 02388711 by Ikeda et al.; Chang Liu, FOUNDATIONS OF MEMS, Chapter 10, pages 1-55 (2006); V. Lehmann, “The Physics of Macropore Formation in Low Doped n-Type Silicon,” J. Electrochem. Soc. 140 (1993), 10, 2836-2843; Vyatkin et al., “Random and Ordered Macropore Formation in p-Type Silicon,” J. Electrochem. Soc. 149, 1, G70-G76 (2002); van den Meerakker et al., “Etching of Deep Macropores in 6 in. Si Wafers,” J. Electrochem. Soc. 147, 7, 2757-2761 (2000); Kanamura et. al., “Electrophoretic Fabrication of LiCoO2 Positive Electrodes for Rechargeable Lithium Batteries,” Journal of Power Sources, 97-98 (2001) 294-297; Caballero et al., “LiNi0.5Mn1.5O4 thick-film electrodes prepared by electrophoretic deposition for use in high voltage lithium-ion batteries,” Journal of Power Sources, 156 (2006) 583-590; Wang and Cao, “Li+-intercalation Electrochemical/Electrochromic Properties Of Vanadium Pentoxide Films By Sol Electrophoretic Deposition,” Electrochimica Acta, 51, (2006), 4865-4872; Nishizawa et al., “Template Synthesis of Polypyrrole-Coated Spinel LiMn2O4 Nanotubules and Their Properties as Cathode Active Materials for Lithium Batteries,” Journal of the Electrochemical Society, 1923-1927, (1997); and Shembel et. al., “Thin Layer Electrolytic Molybdenum Oxysulfides For Lithium Secondary Batteries With Liquid And Polymer Electrolytes,” 5th Advanced Batteries and Accumulators, ABA-2004, Lithium Polymer Electrolytes.
It would be desirable to make three-dimensional electrochemical energy devices that may provide higher energy and power density, while reducing capacity loss due to expansion and contraction of alloy anodes and the resulting disintegration (also known as attrition) of anode material.